The term "hybrid microcircuit" refers to the interconnection and packaging of discrete electronic devices in a thick film or thin film network. In the past, the interconnections have been made by building a circuit or a number of circuits on a ceramic substrate. Recently multilayers of this type have been made by printing alternating layers of copper thick film conductor materials and dielectric materials in a desired configuration on a rigid ceramic substrate such as alumina. Each of the dielectric layers is fired in a nonoxidizing atmosphere to effect densification of the dielectric material without oxidizing the copper conductive material before the next layer is applied. Because the substrate prevents lateral shrinkage, the finished multilayer structure remains flat. Therefore, the thermal coefficient of expansion (TCE) of the thick film conductor and dielectric materials need only approximate the TCE of the substrate in order to obtain relatively flat multilayer structures.
Though the use of thick film pastes is technically adequate, equally satisfactory multilayer structures can be obtained much more economically by the use of the "green tape" method.
This process involves the use of a tape fabricated from 92-94% wt. purity Al.sub.2 O.sub.3 ceramic powder and a flexible polymeric binder, one or more layers of which are metallized with a patterned conductive layer, including punched vias, stacked and then laminated with heat and pressure. After lamination, the multilayer assemblage is cofired in a reducing atmosphere to produce the completed multilayer. As many as forty such alternating layers are used to form high density interconnections for use in various electronic hardware such as computer logic modules.
Since Al.sub.2 O.sub.3 is refractory, the conductor metals must be correspondingly refractory. In the past, W, Mo and Mo/Mn have been used as the conductive layer material for metallic traces and via interconnects. Particularly widely used for multilayers has been the system Al.sub.2 O.sub.3 /Mo-Mn. Despite its advantages, the use of the Al.sub.2 O.sub.3 /Mo-Mn system has several disadvantages. For example, the conductivity of Mo-Mn is too low for modern high speed data processing equipment. Also, Al.sub.2 O.sub.3 /Mo-Mn requires firing temperatures on the order of 1500.degree. C. in a wet H.sub.2 atmosphere to achieve proper densification of the Al.sub.2 O.sub.3. Furthermore, Al.sub.2 O.sub.3 has a temperature coefficient of expansion (TCE) which is twice that of Si. Therefore, large integrated circuit chips often crack when they are bonded to the multilayer substrate because of the mismatch in TCE between Al.sub.2 O.sub.3 and Si. To overcome these problems, Herron et al. in U.S. Pat. No. 4,234,367 and Kymar et al. in U.S. Pat. No. 4,301,324 have proposed the use of green tape in which the Al.sub.2 O.sub.3 is replaced by a glass ceramic having a low crystallization temperature and Mo-Mn is replaced by Cu as the conductive layer material.
Suitable glasses are disclosed to be .beta.-spodumene (Li.sub.2 O.Al.sub.2 O.sub.3.4SiO.sub.2) and cordierite (2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2). Both the spodumene and cordierite sinter below 1000.degree. C.
Using the above-referred materials in the green tape process, the multiple layers are cofired 3-5 hours at 775.degree..+-.10.degree. C. in a steam/H.sub.2 atmosphere to burn out the polymeric binder, after which the H.sub.2 /H.sub.2 O atmosphere is replaced by N.sub.2 and firing is completed at 930.degree.-970.degree. C. to achieve densification of the glass ceramic material. Because of the high sintering temperature, the structure shrinks about 15% and glass ceramic material is crystallized into cordierite ceramic.
In the above-described system, the fine copper powder begins to sinter and shrink when it reaches 400.degree. C. in the firing cycle, whereas the glass ceramic material does not sinter until it reaches 780.degree. C. Because of this difference in the sintering and shrinking characteristics of the two materials, the multilayer tends to incur warping and bowing. For this reason, it has been difficult to produce cordierite/copper multilayer structures with the required degree of flatness. Thus, it would be very desirable to have a copper-based conductive material which would not incur sintering until it reached about 780.degree. C. and which would also have predictable shrinkage characteristics approaching those of green glass ceramic tape, i.e. about 15% shrinkage occurring between 700.degree. and 970.degree. C.
A still further problem with the prior art copper powders has been the unpredictability of the copper powders from batch to batch. This is illustrated in Wolf, J. ed., Powder Metallurgy, Am. Soc. for Metals, Cleveland, OH (1942). In Chapter 31 (pages 323-331) of this publication, J. E. Drapeau, Jr. presents several sintering curves for copper powders under various sintering conditions which are so diverse in shape that it would be extremely difficult if not impossible to predict the sintering characteristics of mixtures of such materials under other sintering conditions or in blends with other copper materials. Thus the availability of copper materials having not only improved but also predictable shrinkage characteristics is an important goal.